1. Field of Invention
This invention is directed to a composition for an acrylic resin that contains crushed natural stone particularly suitable for use as a countertop.
2. Description of the Related Art
Sheets of polished natural stone, such as marble or granite, are used as decorative and functional surfaces in construction applications. However, these products require expensive handling in shaping and finishing and are only available from regions remote from population centers. These factors significantly add to the already high cost of utilizing such materials. Additionally, due to imperfections, cracking and general brittleness can be encountered and the porosity of the material can lead to staining and bacterial growth.
Various synthetic-filled polymeric compositions have been described as useful for end uses such as counter tops, and floors. These synthetic materials are commercially available. Typically, these materials incorporate resin and inorganic fillers and are cured using curing systems that are actuated at room temperature or under similar ambient conditions. Certain synthetic products have become available which provide the appearance of natural marble. These so-called cultured marble products are man-made molded products generally consisting of polymeric resin, highly filled with inorganic particles and pigments. These materials can require gel coats since castings made without gel coats are susceptible to stress cracking and have a tendency to stain. Many of these materials retain the properties of the plastic matrix and are subjected to some hot water whitening due to fissure at the filler/resin bond, and/or undercured or incompletely cured resin (containing residual monomer).
Many attempts have been made to develop products having the pleasing aesthetic appearance of natural products, yet, to be substantially less expensive than the natural products. Most emphasis has been placed on the preparation of synthetic vinyl products, which are relatively inexpensive but have an appearance similar to that of the natural products. Nevertheless, attempts have been made to use natural materials in a manner by which product costs and installation problems are reduced or avoided. One such attempt is described in U.S. Pat. No. 3,278,662 to Mangrum. This reference describes a stone-containing tile product which can be mass-produced, and used to avoid the problems that generally face the installer of terrazzo products. The tile product as disclosed therein contains about 7 to about 25 percent by weight of a thermosetting polyester resin, and about 93 to about 75 percent by weight of stone particles. The components are compressed in a mold and cured; the resulting products are rigid in nature and have deficiencies that are similar to those noted with ceramic and marble tile products.
Another procedure has been commercialized by Breton S.p.A. of Castello di Godego, Italy, which developed a so-called xe2x80x9cBreton Stonexe2x80x9d technology. In this technology, conventional polyester resin precursors are blended at low weight percentages with aggregate to provide a relatively dry mass of material, that is vibro-compacted under vacuum and then cured to yield a rigid tile product. A process used to practice this technology is disclosed by Toncelli in U.S. Pat. No. 4,698,010. A specific polyester resin that can be used in this technology is disclosed by Slocum in U.S. Pat. No. 5,321,055. It is known to add material such as styrene to unsaturated polyester resins and, in fact, they are supplied by chemical suppliers diluted with sufficient styrene for eventual cross-linking of the polyester. The proportions supplied are usually about 70 parts by weight polyester to 30 parts by weight styrene. Such diluted polyesters provide monomer that is required for cross-linking of the polyester. These polyester compositions are disclosed for use as flooring tile. However, flooring tile does not require the heat resistance and flexural properties necessary for a countertop.
The invention is a composition for a surfacing material comprising (a) an acrylic resin matrix with cross-linking controlled by the addition of a tri-functional acrylic monomer and (b) a filler comprised of crushed natural stone.
In typical cross-linking of the unsaturated polyesters of the prior art, cross-linking takes place between the growing styrene homopolymer and the sites of unsaturation in the polyester composition from the maleic anhydride copolymerized into the polyester backbone. The degree of cross-linking can be controlled by the concentration of maleic anhydride in the polyester composition.
In the case of the acrylic functional resins of the instant invention, the molecular weight of the acrylic oligomer is grown through copolymerization between the oligomers and the monomers such as styrene and methylmethacrylate. In the cured polymer the oligomer grows to a higher molecular weight, imparting improved mechanical properties to the polymer. The degree of cross-linking is controlled by the concentration of a tri-functional monomer. The degree of chain extension is controlled by the concentration of ethylenically unsaturated monomer. By controlling the proportions of these monomers, a cured part is made that is less stiff (lower modulus) and has an improved elongation and strain at failure. The stresses built up by the mismatch in thermal expansion coefficients between the polymer and a stone filler employed in the present invention are related to temperature. When a composite is heated, the acrylic polymer is able to relieve these stresses more easily than unsaturated polyester. It is considered that the acrylic polymer possesses a mechanical response with an improved strain to failure. The improved temperature performance with respect to crack initiation is delayed as the polymer is slightly tougher (as characterized by the work to break results) and the stresses that are built up can be absorbed more readily.
Embodiments of the stone-filled acrylic surface material of the instant invention contain, by weight, from 5% to 15% acrylic resin matrix, and 85% to 95% of a natural crushed stone filler. Preferred embodiments contain 6-10% acrylic resin matrix and 90-94% natural crushed stone.
Embodiments of the acrylic resin used as the matrix in the solid surface material disclosed above, may contain from 20% to 65%, by weight, ethylenically unsaturated monomers to promote chain extension in the resin, and from 0.5% to 10% of a tri-functional monomer to promote cross-linking in the resin. A preferred embodiment of the acrylic resin contains from 20 to 65%, by weight, of an ethylenically unsaturated monomer, 65% to 5% of an acrylic oligomer, and 15% to 30% of an acrylic polymer and 1% to 5% of a tri-functional acrylic monomer. The most preferred embodiment of the acrylic resin matrix compositions contain 60%, by weight, ethylenically unsaturated monomer, 19% acrylic polymer, 17% acrylic oligomer, 3% tri-functional acrylic monomer and the remaining 1% made up of minor constituents such as controlled amounts of additives such as initiators, activators, adhesion promoters, pigments, decorative additives, fluidizing agents, viscosity control agents, curing agents, antioxidants, catalyst and the like as are known to those of ordinary skill in the art.
Any ethylenically unsaturated monomer may be used to promote chain extension. Examples of satisfactory ethylenically unsaturated monomers are styrenes, methacrylates and acrylates, or combinations thereof. Preferred monomers are a combination of styrene, alphamethyl styrene, methylmethacrylate, and 2-phenoxyethyl acrylate.
Any tri-functional acrylic monomer may be used to promote cross-linking. The preferred tri-functional monomer is trimethylolpropane trimethacrylate.
The acrylic polymer useful in the present invention is any acrylic polymer that can be formed into high performance decorative solid surface material. Many such acrylic polymers are commercially available. A preferred polymer that meets all of the properties is poly(methyl methacrylate).
The acrylic oligomer is preferably a urethane acrylate. A number of oligomers are commercially available. They range in their backbone chemistry from polyethers to polyesters. They can be aromatic or aliphatic in nature. Some are referred to as epoxy acrylates. The end group functionality of these oligomers can be methacrylate or acrylate. The acrylate functionality is preferred, as its reactivity is more suited towards co-polymerization with the styrene monomers. Preferred oligomers are aliphatic urethane acrylates and methacrylates. Polyesters from epoxy methacrylates and epoxy acrylates and oligomeric propylene oxide, as well as oligomeric ethyleneoxide could also be used. For purposes of this invention, oligomers are short chain polymers where the molecular weight is 500-10 000 g/mole and, depending on the backbone composition, the number of repeating units in the oligomer is from 2 to 22.
Initiators may be any of the free radical producing compounds well known to the art. Peroxides are preferred such as, peroxydicarbonates, peroxyesters, and dialkyl peroxides. Azo type initiators that also thermally decompose may be used and include Vazo(copyright) 52, Vazo(copyright) 64 and Vazo(copyright) 67 (registered trademark of E.I.DuPont de Nemours and Co.).
The coupling agent is a small molecule that aids in the dispersion of a solid particulate material into a liquid medium and are of the following types: silanes, titanates and zirconates. The coupling agent preferred in the present invention is a silane in general, and SILQUEST brand product A-174 manufactured by Witco Corporation in particular.
The crushed natural stone filler may be any fragment of rock or mineral matter found in nature. Satisfactory stone fillers may be quartz, quartzite, marble, granite, feldspar and the like. The preferred stone filler is quartz. The sizes of the stone fillers are typically a combination of 6 mesh, =1.7-5.6 mm, 10 mesh, =0.6-3.35 mm, and 24 mesh =0.15-1.18 mm.
Optionally, any material may be added for a decorative effect. Decorative additives are distinguished from stone fillers primarily by the amount present in the composition. The crushed natural stone filler acts as an aggregate and must be present in the 85% to 95% quantity previously mentioned. Decorative additives such as gemstones, metal flake or filings, micas, seashells, pearls, colored or transparent polymeric particles, mirrored particles and pigments may be added in quantities according to taste or fashion. However, these quantities should never exceed 5% by weight, and preferably, should not exceed 2% by weight. The decorative additives may be thoroughly mixed with the other components or placed on the surface.
Examples 1 and 2 describe the manufacture of embodiments of the acrylic resin matrix material useful for making a stone-filled acrylic surfacing material. All parts and percentages are by weight unless otherwise indicated.